A study of the conversion of ethanol to 1,3-butadiene: effects of chemical and structural heterogeneity on the activity of MgO-SiO₂ mixed oxide catalysts

The ethanol-to-butadiene (ETB) transformation proceeds through consecutive reactions, involving hydrogenation/dehydrogenation, C-C coupling, and dehydration. Uniform active sites are needed to attain high catalytic selectivity. It is a challenge to generate a catalyst containing three kinds of co-operating active sites in high homogeneity. Lacking dehydration activity, basic MgO is active in converting ethanol mainly to acetaldehyde and butanol, whereas the main products obtained over SiO2 catalysts are dehydration products ethylene and diethyl ether. 1,3-Butadiene could be obtained over MgO-SiO2 mixed oxide catalysts, having acidic and basic sites of strength and concentration favoring all three reactions. Silica was either precipitated over the surface of MgO, or wet-kneaded with MgO to get mixed oxide catalysts. More active ETB catalysts were obtained if the MgO component has a higher specific surface area. XRD, EDS, XPS and acidity/basicity examinations showed that Mg atoms got incorporated into the silica phase, generating new Lewis acid surface sites. An amorphous MgO-SiO2 mixed oxide preparation, having the highest surface Mg/Si ratio and atomic homogeneity, had the highest activity and 1,3-butadiene selectivity. The catalyst was obtained by hydrolyzation/condensation/precipitation of an Si,Mg-alkoxide solution within carbon mesopores and burning out the carbon/precipitate material. The catalytic ETB mechanism is discussed.